In the oil industry, wells are often equipped with permanent gauges. Supervisory control and data acquisition (SCADA) systems are also currently used in wells to achieve reduced operating cost and increased recovery factors. As an example, more than 11,000 electrical submersible pumps (ESP) from Schlumberger have been fitted with gauges over the past six years and over 1,000 of these wells have remote monitoring capability using a SCADA system. Despite the large deployment of instrumentation and connectivity, the ability to measure flow rates in real time has lagged and generally remains manual and episodic in nature. Wells are in most cases tested once per month with flow rate measurements entered manually into production databases.
The most common technique used for flow rate measurements uses a test separator which is a vessel into which production is diverted for measurement of the oil, water and gas flow rate of a well. Tests are usually performed on a monthly basis but in many cases the frequency is even less due to logistical reasons. One downside of the use of current test separators is that many wells produce at flow rates below the threshold required to achieve reasonable accuracy. Additionally, flow rate measurement techniques using test separators do not provide the testing frequency, repeatability, or resolution required to create an accurate log of flow rates over time.
Previous attempts to monitor well flow rate in wells with ESPs using downhole gauge data have been made using a differential pressure across the pump and a pump head versus flow curve. This method, while valid in many applications, requires NODAL™ (available from Schlumberger) analysis to calculate an average specific gravity through the pump to convert the measured differential pressure into a differential head. The technique is therefore only valid when steady state conditions apply as this is a prerequisite for NODAL™ analysis. It is also difficult to apply this technique when the gas void fraction (GVF) through the pump is high as this can cause slugging, which means that the well is no longer in steady state.